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[Edward Pinniger]

Is there any way of calculating the amount of ballast weight needed for a R/C sailing model of given hull dimensions + shape, mast height etc.?

I have a hull (actually a converted HMS Bounty) which is a typical 18th/early 19th century merchant ship in form, with a very blunt bow, square sides and stern tapering to a flat transom. Its dimensions are 50cm length (excluding beakhead + bowsprit), 14cm width, 10cm height from keel to deck (the deck will have quite substantial bulwarks, about 2.5cm high) or 6cm from keel to waterline. Doesn't sound like much for a R/C model, but being a merchant ship has a lot more displacement than a yacht of the same length!

I'm planning on building the hull as a two-masted topsail schooner (specifically the Australian merchant ship "Enterprize" of 1830, which I have a plan sheet for and which has an almost identical hull form to Bounty) - when completed the main mast will be about 51cm high from the deck, the fore mast about 45cm. I plan to have only the main + fore sails (and possibly the jib) set - definitely no square sails set - so the model will have a fairly modest amount of sail area.

I'm not very keen on fitting the model with a bulb keel (a permanent one will make the model hard to store/display and I have no experience with making/fitting removable ones) and am wondering if it's practical (given that I'm not intending to sail the model in high winds) to install the ballast inside the hull, in the form of either bagged lead shot, or lead sheet bent to fit the shape of the hull interior? If so, roughly how much would be needed for a model of these hull dimensions?

I've done some tests and have managed to get the hull - with the lower masts installed but nothing else - ballasted down to the waterline and (apparently) very stable with about 12oz of lead, but of course there's no way of telling how stable it will be under sail.

[meatbomber]

Edward, the size of your bounty is very close to my Somers with 60cm on deck 14 width, so i`m a bit baffled that you have so little spare weight left to ballast the ship.
Can you weigth the hull, so we can get an idea of the total displacement to get her to her waterline ?

[Edward Pinniger]

Edward, the size of your bounty is very close to my Somers with 60cm on deck 14 width, so i`m a bit baffled that you have so little spare weight left to ballast the ship.
Can you weigth the hull, so we can get an idea of the total displacement to get her to her waterline ?

Sorry, not sure what I was thinking when I did the first ballast test! I did another test and found that about 1lb 8oz of ballast (this is about all the lead I have!) will get the model near to the waterline. (12oz, as I originally posted, is nowhere near enough). The additional weight of the batteries (4 AAs, about 5oz),should make up the difference. With all this weight in the hull, the model appears very "stiff" in the water (i.e it returns to vertical very quickly when pushed over and let go), but, as I mentioned in my original post, I've no idea how this will translate to stability under sail.

The hull and lower masts weigh about 1lb 8oz (same as the ballast).  It's a typical plank-on-bulkhead kit hull (Mantua, Amati or similar), I cut it down by about 1.5cm to get the right depth for Enterprize (removing the very heavy plywood deck in the process - the new deck is much thinner ply).

Though my model isn't very big by R/C sail standards, Enterprize is a small ship (51ft) so it works out at around 1/35 scale using the Bounty hull, big enough for scale detail to be added without compromising sailing ability. It also has a very simple deck layout. So I thought it'd make an ideal subject for a first attempt at a R/C historic sailing ship.

[meatbomber]

Edward, i had teh same discussion with Brooks Martin on rcgroups when fitting out my Brig Somers. Especially after me observing that tho we had very similar hulls and sail areas i seemd to have to strike sails at unsatisfactorily low winds. We started to compare data:

Brooks Aldebaran measurements:
Freeboard at the center of hull: 3.5 cm to top of bulwark, 2 cm to top of deck.
Fin keel: wood=51x19 cm, net ballast (Pb-vol of container) = 720g.
Righting moment (relative to bottom of hull) = 36 kgcm.
Sail area as Brigantine, max: 4200 cm^2
Sail area as Brig, max: 4000 cm^2

Somers measurements:
Freeboard at the center of hull: 6 cm to top of bulwark, 3,5 cm to top of deck. (freebord as per drawing should be 4,7cm)
Fin keel: Plexi=30x18 cm, net ballast (Pb-vol of container) = 800g.
Righting moment (relative to bottom of hull) = 24 kgcm.  
Sail area max: 3300 cm^2
i´ve added another 200g to the ballast on Somers, that should get me to 30kgcm righting moment

To compare then i divided sail area by righting moment which results in 116,6 cm/kg for Aldebaran, my new ballast results in 110cm/kg for Somes so actually a bit stiffer than Brooks especially  compared to the 137,5 cm/kg i had before adding ballast (i don`t know of a unit of measure that uses cm/kg but it serves the purpose i guess ..

This is by no means scientific but served teh purpose of comparing sailing stiffness between our boats which are very close in hull size but he is using a lot more sail area..
I got the same data for Nevile Wades Square riggers, they are much larger and his stability numbers are much smaller (stiffer). Mine and Brooks can sail with all sails up to about 10kts wind above you have the strike sails and at about 20kts yoou are down to F&A sails only.

hope that helps :)

phillip

[Edward Pinniger]

Thanks for the advice! Maths is definitely not my area of expertise (if anything is) but I'll have a look at those numbers later and see if I can work out what my model should have.

I should add, though, that I'm only planning to sail my model with fore and aft sails set anyway - this is my first attempt atscale R/C sail and I'm trying to keep things as simple as possible.

The good news is that if I slightly increase the size of the cargo hatch, the ballast compartment will be easily accessible with the deck in place, so I don't need to "finalise" the ballast setup before gluing the deck on. Once this is done, I'll try and get a basic temporary rig up as soon as possible so I can see how the boat stands up to winds with the sails in place.

[meatbomber]

Edward, you will definitely need a fin keel... it realy wont work without, or just with scraps of sail set

[Edward Pinniger]

Here are a couple of photos of my "Enterprize" model as it is currently, with spars temporarily taped in place. The Bounty hull was cut down by about 1.5cm with a new deck and bulwarks installed on top, most of the interior bulkheads have been cut down to lighten the hull and provide space for ballast + radio. The bulwarks aren't complete yet (the slightly "wobbly" top edge will be straightened out by the top rails), and the coamings around the deck openings will eventually have removable hatch + deckhouse structures on top.
The rudder (incomplete) is about as large as I can make it and still look reasonably in-scale (actually near twice scale width) but I suspect it'll still need extending on the finished model.

I'm currently working on making the sail gaffs + booms (from wooden paintbrush handles! as are the topmasts + jib boom) so I can get a temporary sail rig up ASAP and test the model out under sail (in the test tank) to check stability. I suspect that I'll need to make a fin keel as you suggest but I'll deal with that problem when I get to it!

[Dueller]

Just in case it's not been mentioned as a tip before. I have made small sail spars etc using dowell then fitting my drill in the vise/vice and using it like a lathe and sanding them to a nice taper. you will have to judge speed and lengh for safety etc.

[Popeye]

I use the  following simple formula to determine the estimated all-up weight of a model:-

 Displacement formula

When:-
A = length* between stem and stern perpendiculars at plan waterline
B = moulded breadth*
C = draft*
D = displacement in kilogrammes
Y= multiplier**
*  in decimetres
** ‘Y’ is a ‘guestimate, which assumes that the underwater volume of the immersed hull is not constant throughout its entire length, i.e. less water is displaced  at the bow (swim) and stern (deadwood) than at midships. I use Y= 0.6 for ‘bathtub’ hulls and Y = 0.5 for yachts or other sleeker craft with their finer underwater lines. By comparision, a square biscuit tin would warrant a multiplier of Y = 1.0

Therefore:-
Displacement (kg) = (A*B*C)*Y

My current project, an 18th Century square rigger, with its heavy displacement ‘bathtub’ hull required loads of ballast to get it down to its plan waterline. I estimated that at !:24 scale its finished  weight would be in the order of 34kg (75lbs) – far too heavy for my weak back to cope with – whereas at 1:32 scale (a less impressive size, but much easier to handle) it would weigh in the order of 14kg (31lbs), still heavy but less likely to give me a hernia than at 1:24 scale.

The brigs’ critical measurements at 1:32 scale were, A = 805mm, B = 235mm, C = 112mm,
therefore estimated displacement (D) was:-
D = (8.05*2.35*1.12)*0.6
D = (21.1876)*0.6
D = 12.713kg (28lbs)

Flotation tests revealed that an all-up weight of 12.25kg (27lbs) was required in order to bring the model down to its plan waterline. This weight comprised:-
Hull, complete         = 7lbs 7oz
Masts, spars sails and rigging                   = 1lb  0oz
Radio inc Rx and Drive batteries                   =2lbs 6oz
Sub total                                                   =10lbs 13oz
Ballast (bolt-on fin)         = 16lbs 5oz
TOTAL WEIGHT         = 27lbs 2oz

No doubt there are more accurate and mathematically complicated methods of calculating  total displacement though as far as I’m concerned my rough and ready method is accurate enough for my needs and I’ll continue to use it  for future models.

 

[Edward Pinniger]

Thanks again for the advice, it should come in handy when I get to the final ballasting stage. I still need to finish the spars + make the sails so I can find out the model's stability under sail. Hopefully the hull's wooden keel will be strong enough to take the weight of a bolted-on fin keel - it's heavily reinforced with Cascamite wood glue (which I also used to seal the hull planks, inside and out)

Your 18th c. brig model looks great! Is it based on a particular ship/class, or freelance? I was originally intending to convert my Bounty hull to something like this, but when I found the Enterprize plans (in an issue of Model Shipwright) I thought it would be a better choice for a novice, as it has such a simple deck layout (no guns etc. to get in the way of working lines) and is a schooner rather than a square-rigger.

What materials did you build your hull from? 7lbs sounds quite heavy for an unballasted 80cm hull without masts/spars (not that I'm any expert judge of this sort of thing!)

Just in case it's not been mentioned as a tip before. I have made small sail spars etc using dowell then fitting my drill in the vise/vice and using it like a lathe and sanding them to a nice taper. you will have to judge speed and lengh for safety etc.

I'm planning to make the square sail yards for the foremast this way, as they need to be tapered at both ends. But brush handles are still a great source of tapered wood for spars, especially the large bristle brushes with unpainted handles, which you can get at around £2-3 (or even less) per 10. The largest brushes are too thick for most spars, but are still useful for their intended purpose (applying sealant, glue, etc. - they're too coarse for paint or varnish)!

[Popeye]

For Edward Pinniger

In response to your questions:-

Weight of hull (7lbs) includes heavy duty  servos, Rx and Drive batteries, electric motor, ESC,  masts,  spars, running and standing rigging, sails, plus all deck fittings inc., eight cannons and six swivel guns (all removable to eliminate risk of snagging sheets), removable hatches, capstan, winch etc.- in fact everything bar the kitchen sink!

Hull is plank on frame utilising 3/8” x 1/8” cedar planking on sacrificial balsa frames. Most frames were removed when the hull was fully planked and after being covered externally  with lightweight GRP tissue and GRP resin. Interior was also resined after removal of balsa frames.   
Deck is planked with 5mm x 1.5mm lime stripwood.

 ‘HMS Thames’,  is a two masted brig based on lines taken from ‘HMS Supply’  a yard transport built at on the Thames at Rotherhithe in 1759. ‘Supply’ was re-fitted in 1787 to enable her to accompany the first convict fleet to sail for Australia. Her crew were the  first Europeans to step ashore at Port Jackson thereby establishing the first British colony in Australia, an event which was commemorated by the depiction of the vessel on Australia's Millennium issue $10 note. Another historical note - 'Supply' was so fast that the Fleet's commodadore, Capt, Phillips, transferred his Flag to 'Suppyl', threby giving her the distinction of being the Navy's oldest and smallest flagship of all time.

Email  me via Mayhem if you’d like some construction pics and/or sail servo sheeting arrangements.

[Popeye]

For Edward Pinnagar.

Whoops re my earlier reply. I got carried away (never respond to a message after a long and tiring drive without first putting one's brain in gear!)  and gave you a bum steer regarding hull weight which is indeed 7lbs 7oz without anything else.

Yes it is heavy but it's also very strong and is unlikely to hog, sag or twist bearing in mid the fact that a further 16lbs of bolt on ballast (fin) was needed to get it down to it's waterline

[Edward Pinniger]

Thanks for the info + more photos of your very nice looking brig model!
It's interesting that my model is also based on a ship of historical significance in Australia (Enterprize carried some of the founders of the city of Melbourne). I picked this subject simply because it was the right shape for my Bounty hull, and a suitably simple rig and deck layout for a beginner! I was originally going to build it as Enterprize but have now decided to make it as a generic early 19th century coasting schooner, which will give me more freedom with painting + detailing the model - The 1:1 Enterprize replica has a natural wood finish on most of the hull which isn't very suited to my hacked-about, patched-up hull! I will be painting mine black with a "false gunport" stripe on the bulwarks.

If it turns out OK (and is sailable) I'll attempt something more ambitious as a next project, probably a small naval vessel (brig or sloop) around 3ft LOA, using a scratchbuilt hull.

[Edward Pinniger]

After some basic stability tests (in a water-filled wheelbarrow with a household electric fan to provide wind) which seemed to go OK, I gave my model a test sail at Black Park yesterday and was surprised to find that it sailed OK with no stability problems.





(The upper hull + bulwarks have since been painted!)

It coped with reasonable strength winds from all directions and even in gusts the water barely came up to the deck level. I was very surprised by this - I assume the very deep, slab-sided hull (compared to a yacht) combined with the minimal sail area, meant that the internal ballast was sufficient.
Total ballast weight including the batteries is about 2lb.

Sailing performance wasn't spectacular, but it did sail, which was more than I expected! The rudder (about twice scale size, but still not very big) also gave acceptable steering ability, though I may try using a removable rudder extension next time I sail it.

Would I be able to get away with a jib sail as well?

[Little Rascal]

I use the  following simple formula to determine the estimated all-up weight of a model:-

 Displacement formula

When:-
A = length* between stem and stern perpendiculars at plan waterline
B = moulded breadth*
C = draft*
D = displacement in kilogrammes
Y= multiplier**
*  in decimetres
** ‘Y’ is a ‘guestimate, which assumes that the underwater volume of the immersed hull is not constant throughout its entire length, i.e. less water is displaced  at the bow (swim) and stern (deadwood) than at midships. I use Y= 0.6 for ‘bathtub’ hulls and Y = 0.5 for yachts or other sleeker craft with their finer underwater lines. By comparision, a square biscuit tin would warrant a multiplier of Y = 1.0

Therefore:-
Displacement (kg) = (A*B*C)*Y

My current project, an 18th Century square rigger, with its heavy displacement ‘bathtub’ hull required loads of ballast to get it down to its plan waterline. I estimated that at !:24 scale its finished  weight would be in the order of 34kg (75lbs) – far too heavy for my weak back to cope with – whereas at 1:32 scale (a less impressive size, but much easier to handle) it would weigh in the order of 14kg (31lbs), still heavy but less likely to give me a hernia than at 1:24 scale.

The brigs’ critical measurements at 1:32 scale were, A = 805mm, B = 235mm, C = 112mm,
therefore estimated displacement (D) was:-
D = (8.05*2.35*1.12)*0.6
D = (21.1876)*0.6
D = 12.713kg (28lbs)

Flotation tests revealed that an all-up weight of 12.25kg (27lbs) was required in order to bring the model down to its plan waterline. This weight comprised:-
Hull, complete         = 7lbs 7oz
Masts, spars sails and rigging                   = 1lb  0oz
Radio inc Rx and Drive batteries                   =2lbs 6oz
Sub total                                                   =10lbs 13oz
Ballast (bolt-on fin)         = 16lbs 5oz
TOTAL WEIGHT         = 27lbs 2oz

No doubt there are more accurate and mathematically complicated methods of calculating  total displacement though as far as I’m concerned my rough and ready method is accurate enough for my needs and I’ll continue to use it  for future models.

Hi Popeye!

The method of rough calculation you described is more or less what full size naval architects use for preliminary displacement calculations.

The 'multiplier' you refer to is called the Block Coefficient. Exactly as you say, the block is the load waterline length x waterline beam x hull depth. The block coefficient used is based on experience or measurement of similar hull types. Usually around 0.35-0.55. Obviously the finer the hull the lower the coefficient. At 0.6 you obviously have very bluff bows and full sections.

The only other difference is the letters used in the formula:

LWL - Load waterline length
BWL - Waterline beam
D - Hull depth (excluding keel and appendages - not draught)
delta (font won't do greek letters!) - Displacement
Cb - Block coefficent

Displacement = LWL x BWL x D x Cb

If you invert the formula you can work out the block coefficient from an existing model if you know its LWL, BWL and Hull depth as well as its actual displacement. This enables you to assess existing designs and build up parameters that help you estimate CB on a new design.

Cb = delta/(LWL x BWL x D)

The formulas work best in cm and kg because 1cc = 1 gram. To use feet/lbs a correction factor is needed.

To calculate the displacement more accurately you need to use Simpson's Multipliers method. Basically measuring the area of each section to calculate the volume displaced. The great thing about this method is it also tells you where the centre of buoyany lies, and therefore where you need to put the center of gravity.


Meatbomber - I'm interested to know how you measured the righting moments you mentioned above? Did you do it empirically in a test tank or something or from a lines drawing?

[meatbomber]

the righting momenbt was just weight of the ballast(-displacement of teh keel) * length of the keel  we used from the waterline for comparison purpose, center of boyancy would be better but that`s reasonably close anyway... it`s just a rough comparison of boats of about the same size / hull, as it doesn`t take into account any form stability at all.